Some alternative schemes for estimating the channel \(\bf {G}\)  between the RIS and the BS were proposed [1]}, [2]}. In [1]}, two users (\(U_1\)  and \(U_2\) ) are deployed near the RIS to assist its channel estimation. The uplink cascaded channels \({\bf {H}}_{1}\)  for user \(U_1\) , \({\bf {H}}_{2}\)  for user \(U_2\) , and the \(U_1\) -RIS-\(U_2\)  cascaded channel between the user \(U_1\)  and the user \(U_2\)  via the RIS are estimated based on the pilot symbols transmitted by the two users, respectively. After that, the entries for BS-RIS channel \(\bf {G}\)  can be calculated based on three estimated cascaded channels mentioned above. By utilizing the long-term channel averaging prior information [2]}, the large timescale channel \(\bf {G}\)  can also be estimated based on the channel matrix calibration.
